Methyl salicylate has been reported as repelling the black bean aphid, Aphis fabae, and cereal aphids including the grain aphid, Sitobion avenae, and also inhibiting attraction to their host plants. See J. Chem. Ecol. 20, 2565-2574 (1994) (Pettersson et al.) and J. Chem. Ecol. 20, 2847-2855 (1994) (Hardie et al.). It was suggested that this repellency arose from the relationship of methyl salicylate with salicylic acid and inducible plant defense mechanisms, with the metabolite methyl salicylate acting as a volatile and thereby external signal; the presence of methyl salicylate signalled that chemical defense was induced, and the otherwise attractive host plants were thus perceived as unsuitable hosts by the aphid pests.
More recently, it has been shown that methyl salicylate also acts as an airborne signal mediating plant pathogen resistance. See Nature 385, 718-721 (1997) (Shulaev et al.). Methyl salicylate was initially identified by Pettersson et al. as an aphid semiochemical (for example a behaviour-controlling chemical or a signal otherwise influencing the physiology of the organism) by gas chromatography (GC) coupled directly to a single cell recording (SCR) from the olfactory organs on the antenna. Subsequently, more than thirty species of insects, both plant-feeders and their natural enemies, from four orders have been found to possess highly specific and sensitive olfactory receptors for this compound.
WO-A-91/19512 (Washington State University Research Foundation) discloses a method of inducing plant defense mechanisms using jasmonic acid or its esters, or substituted derivatives thereof. The compound induces the production of plant defense proteins, such as proteinase inhibitors, and can promote insect, pathogen or viral resistance in plants by inducing the expression of plant defense genes. Plants may be contacted with the compound by direct application to plant tissue or by airborne transmission of the compound. The expression of plant defense proteins is useful in protecting the plants from the effects of insect attack, but does not prevent the insects in question from attacking the plants. The plants, together with any adjacent plants, will continue to be attacked by predators. The effect moreover is generally short-lived and disappears after removal of the stimulus.
Jasmonic acid and methyl jasmonate, along with a number of other materials, are also discussed by Karban and Baldwin in Induced Responses to Herbivory 12-46 (The University of Chicago Press, Chicago, 1997).
Another material, cis-jasmone, is well known as a volatile component of plants and its release can be induced by damage, for example during feeding on cotton by lepidopterous larvae. See J. Chem. Ecol., 21, 1217-1227 (1995) (Loughrin et al.). It is a fragrant material and has often been used for this desirable property. U.S. Pat. No. 4,788,164 (Che et al./Hoechst Celanese Corporation) discloses a sustained release composition including a fragrance or an insect repellent. One example (example IV) uses a solution containing jasmone to impart the odour of jasmine.
U.S. Pat. No. 5,665,344 (Pair et al./The United States of America as represented by the Secretary of Agriculture) indicates that compositions of cis-jasmone were found to attract adult Lepidoptera. The cis-jasmone may be used alone or in combination with one or more other volatiles of the Japanese honeysuckle flower, particularly linalool and/or phenyl-acetaldehyde. By attracting the adult Lepidoptera to attracticidal baits and/or field traps, the attractants are said to be useful for the control and monitoring of these agricultural pests. The cis-jasmone may be combined with an insect toxicant or pesticide to kill these pests.
We have now discovered that cis-jasmone also has direct signalling roles with plant-feeding aphids, in attraction of aphid predators and parasitoids, and as an airborne signal inducing production of volatile plant components, including the monoterpene (E)-β-ocimene, that stimulate foraging by parasitoids. This signalling role is qualitatively different from that of the biosynthetically related methyl jasmonate and gives a long-lasting effect after removal of the stimulus. In contrast to what the prior art suggests, it may be used to attract insects which are beneficial to the plants concerned or to repel undesirable insects.